Method of measuring magnetic field characteristics of magnetic materials

ABSTRACT

Bodies of a ferrimagnetic material are disposed within small cavities provided in a magnetometer. The magnetometer is initially calibrated utilizing an electromagnet to produce an applied magnetic field of either known field strength, H DC , or known field distribution. Radio frequency energy is fed equally to each cavity and each material and results in resonance frequency output signals which may be coupled to a visual display. When the calibrating magnetic field is of known uniform strength, H DC , the orientation of the bodies is changed until each of the resonance signals have a frequency substantially given by f c  =γH DC  where γ=2.8 MHz/oersted. When the calibrating magnetic field has a known field distribution the orientation of bodies is changed until such resonance signals have frequencies related to one another in accordance with the relative magnetic field strengths passing through the cavities. After calibration, the electromagnet is replaced by a test fixture including a magnet, the magnetic properties of which are to be measured. Radio frequency energy signals are reintroduced into the cavities. The frequency (f M ) of each of the output signals is then measured. The strength of magnetic field (H M ) of the permanent magnet passing through the cavities is calculated in accordance with H M  =f M  /γ. The distribution of the magnetic field of the magnet under evaluation is determined by comparing frequencies of the output signals.

BACKGROUND OF THE INVENTION

The invention relates generally to methods and apparatus for measuringmagnetic field strengths and field uniformity of permanent magnets.

As is known in the art, a magnetometer is an instrument for measuringthe strength and uniformity of a magnetic field. A prior art measurementtechnique involves the use of Hall effect probes operating with director differential readouts to measure magnetic field strengths and theuniformity of such magnetic field strengths. The magnetic field strengthis measured by the amount of current produced by the Hall effect probewhen such probe is inserted into a magnetic field being measured. TheHall effect is defined in accordance with the equation F=eBv; whereF=the force; e=the electric charge; B=the magnetic field; and v=thevelocity of the electron charge. The Hall effect technique is based onthe phenomenon which occurs when a thin sheet of metal or semiconductorwith an electric current flowing along its length is placed at rightangles to a magnetic field. An electromotive force is developed which isat right angles both to the direction of the magnetic field and theelectric current. The Hall coefficient associated with the Hall effecttakes place when the electromotive force results in a transversepotential gradient in a conductor or semiconductor. While such probe maybe accurate in measuring magnetic field strengths and uniformities insome applications, in other applications such probes do not provide therequisite accuracy.

SUMMARY OF THE INVENTION

A method of and apparatus for measuring magnetic field properties of apermanent magnet. Initially, at least one body of a ferrimagneticmaterial, such as yttrium iron garnet (YIG) is disposed within a smallcavity in apparatus such as a magnetometer for, illustratively, magneticfield strength measurement of a permanent magnet. During an initialcalibration the magnetometer is positioned within a known appliedmagnetic field (H_(DC)) of, for example, an adjustable calibratingelectromagnet with the field traversing the cavities. Radio frequencyenergy having a linearly modulated range of frequencies is fed to theYIG-cavity combination and a resonance frequency output signal isproduced in the cavity and may be coupled out of such cavity for displayon an oscilloscope. The time base or X-axis of the oscilloscope isgenerated to correspond to the frequency of radio frequency energy fedto the YIG-cavity combination. To measure magnetic field strength, thebody of YIG material is orientated within the cavity until a resonancefrequency output signal is produced having a frequency f_(c) defined bythe equation: (f_(c) =γH_(DC)); γ=the gyromagnetic ratio (2.8MHz/oersted for YIG); and H.sub. DC =the applied magnetic field inoersteds. Such calibrated orientation indicates that the internalinduced demagnetization field is effectively zero.

After the apparatus is calibrated the sphere is fixed in the orientatedposition as established by the calibration procedure. The apparatus isnow utilized for evaluating magnetic field strength of a magnet by firstremoving the calibrating electromagnet and substituting in its place themagnet to be measured. Similary modulated radio frequency energy signalsare fed into the cavities. The resonance frequency (f_(M)) of the outputsignal now produced is a measure of the magnetic field strength (H_(M))in accordance with the equation: (H_(M) =f_(M) /γ) which may be readilysolved.

For a measurement of magnetic field distribution, the field distributionof the calibrating magnet is established. The spheres are oriented inthe cavities to produce output signals having a frequency distributionrelated to the magnetic field distribution of the calibrating magnet.For example if the calibrating magnet produced a magnetic field having afield strength which linearly decreases across the cavities the spheresare orientated so that the frequencies of the output signals decreasedlinearly from cavity to cavity. With the spheres in the orientatedpositions the calibrating magnet is replaced with a magnetic fielddistribution of which is to be evaluated. The variations, in thefrequency of the output signals are measured to provide an indication ofmagnetic field distribution of the magnet being investigated. In theexample above if the plural output signal frequencies vary linearly fromcavity to cavity the field distribution of the magnet underinvestigation is known to vary linearly across the cavities.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features of the invention will become moreapparent after consideration of the following detailed descriptiontogether with the accompanying drawings in which:

FIG. 1 is a diagrammatic view of the apparatus used to calibrate themagnetometer and measure the magnetic field of the permanent magnet;

FIGS. 1A, 1B and 1C are pictorial representations of the displayedoutput detector signals on the screen of an oscilloscope, FIGS. 1A and1B being pictorial representations before and after, respectively,adjustment of the spheres in the cavities for calibration of themagnetometer and FIG. 1C being a pictorial representation afterinsertion in the magnetometer of a magnet the field strength of which isbeing measured;

FIG. 2 is a top view of a magnetometer in accordance with the invention;

FIG. 3 is a fragmentary pictorial view of a portion of the magnetometer,partially in cross section, illustrating a YIG sphere in a cavity andradio frequency coaxial conductor structure; and

FIG. 4 is a side view of a test fixture for measurement of a permanentmagnet using the method and apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1 equipment used in the method of calibration ofmagnetometer 16 and in the method of measurement of the magnetic fieldof a magnet will be described. A voltage controlled radio frequencyoscillator 10 is provided, illustratively a backward wave tube, fed by amodulator 13 which generates a sawtooth waveform 11, periodicallylinearly varying from zero volts to v₁ volts as shown. The sawtoothwaveform 11 is fed to the radio frequency oscillator 10 which produces aradio frequency signal having a frequency linearly modulated through arange of frequencies f₁ to f₂, preselected on the basis of the end usefor which the magnet to be measured is intended. The waveform 11 is alsofed, via line 60, to the time or x axis of oscilloscope 20. Thereforethe x axis of the oscilloscope 20 may be considered as the "frequencyaxis." The radio frequency signal produced by oscillator 10 is fed tocoupler 12. One branch of the coupler 12 is coupled to the (Y₄) verticalaxis of oscilloscope 20 by line 18 through a narrow band filter, here afrequency wavemeter 22 which is tuned to a substantially precisefrequency (f_(c)) selected in the range f₁ to f₂ based on the intendeddevice end use, and a video detector 24, as shown. Therefore as thefrequency of the oscillator 10 passes through the frequency (f_(c)) apulse 25 is produced by detector 24 (shown in FIG. 1A), such pulse 25occurring at a position along the x axis of the screen 21 of theoscilloscope 20 which corresponds to the frequency (f_(c)). Anotherbranch of coupler 12 is connected to a 3:1 power divider 14 and aplurality of signals of equal phase and equal amplitude are transmittedto coaxial connectors 52 of magnetometer 16. The upper and lowersurfaces 29, 31 of magnetometer 16 are disposed orthogonally to themagnetic field produced by the North (N) pole 15 and South (S) pole 17,of a calibrating electromagnet 19. The current for the electromagnet 19is adjusted by variable (DC) source 23 until a predetermined magneticfield of known strength (H_(DC)) is produced across the surfaces 29, 31of the magnetometer 16. Magnetometer body member 30 is provided with aplurality of small cavities 32, 34 and 36 and small bodies of ferro- orferrimagnetic material, such as YIG spheres 38 are mounted in eachcavity (as shown in FIG. 3).

Referring now to FIGS. 2 and 3 of the drawings, wherein like referencenumerals designate like parts shown in FIG. 1 the illustration showncomprises a top view of magnetometer 16. Body member 30 comprises asingle brass metal plate, here 0.060 inches thick, having an uppersurface 29 and a lower surface 31 and defining microwave cavities 32, 34and 36. Small YIG or gallium-doped YIG bodies, such as spheres 38, hereapproximately 0.010 inches in diameter are mounted in each cavity ondielectric rods 40 with screw means 42 for adjustment of the positioningof the rods and hence the position or orientation of the spheres 38within the cavities, 32, 34, 36. Each sphere 38 is bonded to thedielectric rod 40, here of boron nitride, by a suitable epoxy. YIGspheres 38 are of a single crystal material and the preferred magneticfield alignment is along the "easy" crystalline axis direction: For theselected material such "easy" magnetization axis is along the bodydiagonal 111 crystal axis. One method of affixing the sphere 38 to rod40 is referred to as magnetic levitation and comprises alternatelyapplying a magnetic field from opposing directions along a horizontalplane until the sphere ceases to rotate, indicating alignment along the[111] axis. Rod 40 is then introduced vertically and axially along theperpendicular or face diagonal 110 crystalline directional and bonded tothe sphere. Numerous other techniques for establishing the preferredorientation of the sphere 38 on rod 40 are well established anddescribed in the art. See for example the text Microwave Filters,Impedance Matching Networks and Coupling Structures, Matthaei, Young andJones, McGraw Hill, Inc. 1964, pps. 1040-1043, inclusive. Rod 40 has adiameter of approximately 0.005 to 0.020 inches and a hole is drilled inmember 30 to accommodate the rod 40 and sphere 38 when inserted. Agenerally cylindrical dielectric insert 41 is provided within thepassageway after the rod 40 and sphere 38 are inserted as shown in FIG.3.

The radio frequency signals from power divider 14 (FIG. 1) areintroduced to the cavity-sphere combinations by input coaxial connectors52 and coaxial transmission lines. Each input coaxial waveguide linecomprises a center conductor 44, preferably of copper, a silver-copperouter conductor 45 connected to connector 52 and a dielectric 47 such asTeflon (FIG. 2). Center conductors 44 pass from connector 52 to the wallof cavities 32, 34, 36 and are insulated from the body member 30 bydielectric 47, as shown. One end of center conductor 44 is secured forexample by soldering to the cavity walls as at 48, as shown in FIG. 3.Output radio frequency signals from the cavity-sphere combinations arecoupled by output coaxial waveguide transmission lines including acenter conductor 46, insulated by dielectric 51 from body member 30 andthe output conductor 49 of the coaxial conductors 54. The centerconductors 46 are terminated similarly by soldering the ends thereof asat 50 to the cavity walls. The output radio frequency signals areconverted by detectors 26 to video signals and transmitted by lines 28for display on the Y₁, Y₂, Y₃ vertical axis of the screen 21 ofoscilloscope 20 (FIG. 1).

In operation, referring again to FIGS. 1, 1A and 1B, the frequency ofthe modulated radio frequency signals transmitted from oscillator 10 tothe cavities 32, 34, 36 periodically passes through frequency (f_(c))and calibration spikes 25 appear on the oscilloscope screen 21 aspreviously described. The applied magnetic field (H_(DC)) produced bythe calibrating electromagnet is adjusted by variable (DC) voltagesource 23 so that such field is (H_(DC) =f_(c) /γ) and is uniform acrossthe cavities 32, 34, 36. The output radio frequency signals from thecavities 32, 34, 36 are coupled by connectors 54 to matched detectors 26which convert the signals to video signals for transmission by lines 28for display on the Y₁, Y₂, Y₃ vertical axes respectively, ofoscilloscope screen 21. Such signals appear as pulses 19, 19' and 19" asshown in FIG. 1A prior to calibration. It is noted that in the generalcase, prior to calibration, the signals coupled out of the cavities 32,34, 36 will have frequencies other than the frequency (f_(c)). This isdue to the fact that ferromagnetic resonance of the magnetized YIGmaterial when radiated by radio frequency energy is influenced by: (1)the external and internal magnetic field environment; (2) magneticanistrophy and (3) crystallographic axis orientation with respect to thefield applied by the calibrating magnet. The output signals, initiallypulses 19, 19', 19" (FIG. 1A) in the presence of the calibratedexternally applied magnetic field (H_(DC)) produced by calibratingelectromagnet 19 induces an internal demagnetization field N (4πM)opposing applied external field (H_(DC)); where (4πM) is the magneticsaturation and (N) is the demagnetization factor, (here 1/3 for smallspheres) for particular material having a particular orientation andgeometry as is known M is related to the orientation of thecyrstallographic axis of the spheres relative to the direction of themagnetic field and is therefore also related to the orientation of thespheres in the cavities. The resultant internal field is [H_(I) =H_(DC)-N (4πM)]. Further detailed information and explanatory matter may befound in the text Microwave Ferrites and Ferrimagnetics, Lax and Button,McGraw-Hill Box Co. Inc. New York, N.Y. 1962, pgs. 80-84 and 157-168inclusive.

In accordance with the invention and referring to FIGS. 1A and 1B, theorientation of spheres 38 is adjusted until the internal demagnetizationfield is zero and (H_(I) =H_(DC)); where (H_(I)) is the internalmagnetic field and (H_(DC)) is the external applied field of thecalibrating magnet. This condition is indicated when the pulse 19, 19'and 19" are displayed as sharp pulses or pips having a relatively narrowbandwidth at substantially the same reference frequency (f_(c)). Suchpulses 19, 19' and 19" will then be in substantial alignment with pulses25, as shown in FIG. 1B, and indicated by the vertical dashed line. Thecondition, shown in FIG. 1B, results therefore, when the spheres 38 areproperly orientated in cavities 32, 34, 36 and each of the outputsignals has the same resonance frequency (f_(c)) represented in theequation (f_(c) =γH_(DC)). The magnetometer is now considered to becalibrated and the orientation of the spheres 38 is now fixed with thecavities 32, 34, 36, as by applying a suitable bonding epoxy to thescrew means 42, refer to FIG. 3.

After the magnetometer 16 is calibrated, the magnetic field measurementof a permanent magnet sample, such as samarium cobalt 62 is performed bythe removal of the adjustable electromagnet 19 and substitution of atest fixture 64, shown in FIG. 4, containing the magnet 62. Test fixture64 comprises a yoke 66, with pole piece 68 and provides a return path(arrow 63) for the magnetic-field of magnet sample 62 along with thecalibrated magnetometer 16. Surface 69 of the magnet 62 to be measuredcontacts the soft iron pole piece 68 and the magnetometer 16 is againlocated with the surfaces 29 and 31 orthogonal to the magnetic field ofmagnet 62. A temperature compensating ferrite shunt 70 may be employedin order to permit selection of magnets for operation over an extendedtemperature range. Shunt 70 is an annular ring fitted partially overmagnet 62 and partially over pole piece 68.

The periodically linearly modulated radio frequency energy fromoscillator 10 (FIG. 1) is reintroduced into the magnetometer cavities32, 34, 36 with the spheres 38 fixed in the calibrated orientatedposition, described above. Referring now to FIG. 1C, the resonancefrequencies of the output signals are determined by measuring theposition of the pulses 27, 27', 27" on the screen 21 of oscilloscope 20.The degree to which the pulses 27, 27', 27" are displaced from thereference pulse 25 provides a ready indication of whether the magnet 62being tested is acceptable or is not acceptable. The frequencies of theoutput signals are desirably similar to the frequencies (f_(c)), andpulse 25, for the calibrated spheres in cavities 32, 34, 36 to therebyindicate the condition for the magnetic field distribution and be usedin measurement of magnet field strength of the magnet sample 62 beingmeasured relative to the properties of the calibrating magnet.

The measured frequency provides a determination of the magnetic-fieldstrength passing through each of the sphere-cavity combinations. Hence,if the frequency of the signal producing pulse 27 is determined as(f_(m1)), the strength of the portion of the magnetic field passingthrough cavity 32 or (H_(M1)) is derived from the equation (H_(M1)=f_(M1) /γ). Similarly the strengths of the portions of the magneticfield passing through cavities 34, 36 may be derived as (H_(M2) =f_(M2)/γ) and (H_(M3) =f_(M3) /γ) where (f_(M2)) is the frequency of theoutput signal emanating from cavity 34 and (f_(M3)) is the frequency ofthe output signal emanating from cavity 36. Plural bodies and cavitiesin the calibrated magnetometer, therefore, are indicative of magneticfield distribution across the permanent magnet being measured. Magneticfield strength of the permanent magnet being measured, however isachieved with at least one such body of a ferromagnetic resonancematerial in at least one cavity of a calibrated magnetometer.

In an alternative embodiment of the invention the apparatus described inconnection with FIG. 1 may be used to determine the magnetic fielddistribution of a magnet. Here, during calibration the magnetic fieldpassing through the cavities 32, 34, 36 is adjusted to provide a knownfield distribution. For example, consider that a calibrating magnet isused which provides a magnetic field distribution which increaseslinearly across the cavities 32, 34, 36. In other words the calibratingelectromagnet produces a field of H_(o) through cavity 32, 1.1H_(o)across cavity 34 and 1.2H_(o) across cavity 36. The orientation of thesphere in the cavities is adjusted so that the output signal produced bycavity 32 has a frequency f_(o), the output signal produced by cavity 34has a frequency 1.1f_(o) and the output signal produced by cavity 36 hasa frequency 1.2f_(o). The spheres are now fixed in the orientation andthe calibrating electromagnet is replaced with a magnet, the magneticfield strength distribution of which is to be measured. The frequenciesof the output signals of cavities 32, 34, 36 are then measured. If thefrequencies of the output signals of cavities 32, 34, 36 increaselinearly then the field distribution of the magnet is known to alsoincrease linearly across the cavities. Alternatively, during calibrationthe spheres may be orientated so that the frequencies of the outputsignals of cavities 32, 34, 36 are all equal with the calibrating magnetproducing a linearly increasing magnetic field distribution across thecavities. Then, if a magnet, the magnetic field distribution of which isto be measured, is inserted in place of the calibrating magnet, itfollows that if such magnet has a uniform field distribution across thecavities then each one of the frequencies of the signals produced bycavities 32, 34, 36 will be equal to one another. It is noted, however,that such frequency may not be the same as the frequency produced withthe calibrating magnet unless the actual field passing through thecavities is the same for both the calibrating magnet and the substitutedmagnet.

It follows then that in the more general case the magnetometer 16 may beused to measure the field distribution of a magnet whether such magnethas a uniform, linear or non linear distribution. For example, if it isdesired that the magnet have a particular field distribution acalibrating magnet is provided to produce this desired fielddistribution across the cavities. The spheres are then adjusted so thatoutput signals are produced in the cavities to provide an indication ofthe field distribution; for example, the spheres may be oriented so thateach of the output signals have the same frequency. The calibratingmagnet is then replaced with the magnet which is to be measured. If themagnet has the desired field distribution across the cavities thefrequencies of the output signal will be equal to each other. A measureof the deviation of the field distribution from the desired distributionis determined by the relative frequency differential between thefrequencies of the output signals.

This completes the description of the method and apparatus for measuringmagnetic field characteristics. It is understood that other materialsmay be employed in the magnetometer, such as ferrite, ferrimagnetic orferromagnetic materials along with such metallic materials as nickel aslong as such materials have the capability of producing ferromagneticresonance in the presence of a magnetic field and radio frequencyenergy. Additionally, various modifications in the peferred embodiments,illustrated and described herein, may also be made by those skilled inthe art without departing from the spirit and scope of the invention asexpressed in the accompanying claims. Therefore, all matter shown anddescribed is to be interpreted as illustrative only and not in alimiting sense.

What is claimed is:
 1. A method of measuring a magnetic field comprisingthe steps of:(a) passing a magnetic field of a calibrating magnetthrough a cavity in an apparatus having disposed therein a materialadapted to produce ferromagnetic resonance; (b) coupling radio frequencyenergy into said cavity; (c) positioning said material within saidcavity to produce a first output signal having a frequency related tothe magnetic field of the calibrating magnet passing into the cavity andthrough said material; (d) passing a second magnetic field to bemeasured into the cavity and through the positioned material; (e)coupling radio frequency energy into said cavity to generate a secondoutput signal; and (f) comparing the frequency of the first outputsignal with the frequency of the second output signal to provide ameasure of the second magnetic field.
 2. A method of measuring magneticfield distribution comprising the steps of:(a) placing apparatus havinga plurality of cavities with materials adapted to produce ferromagneticresonance disposed in each cavity in a magnetic field of a calibratingmagnet having a known magnetic field distribution across the cavities;(b) coupling radio frequency energy into the plurality of cavities andthrough the materials; (c) positioning the material within said cavitiesto produce a plurality of first output signals having frequenciesrelated to the relative magnetic field distribution of the calibratingmagnet passing through the cavities; (d) substituting a magnet whosemagnetic field distribution is to be measured for the calibratingmagnet; (e) coupling radio frequency energy into the plurality ofcavities and through the positioned materials to generate a plurality ofsecond output signals; (f) measuring the relative frequencies of theplurality of second output signals across said cavities and comparingthe frequencies of such signals with the relative frequencies of theplurality of first output signals to determine the magnetic fielddistribution of the magnet being measured.
 3. A method of measuringmagnetic field strength comprising the steps of:(a) placing apparatushaving a cavity with a material adapted to produce ferromagneticresonance disposed therein in a magnetic field of a calibrating magnethaving a predetermined magnetic field strength (H_(DC)); (b) couplingradio frequency energy into said cavity and through the material togenerate a first output signal having a frequency related to theorientation of the material in said cavity; (c) positioning the materialto a calibrated orientation position resulting in first output signalshaving a frequency (f_(c)) defined by the equation (f_(c) =γH_(DC))where γ=the gyromagnetic ratio of the material (2.8 MHz/oersted); (d)substituting the magnetic field of a magnet having an unknown magneticfield strength (H_(M)) with the material in a calibrated position forthe pedetermined magnetic field strength of the calibrating magnet; (e)coupling the radio frequency energy into the cavity and the material inthe calibrated position to generate a second output signal having ameasured frequency (f_(M)); and (f) determining the magnetic fieldstrength (H_(M)) in accordance with the equation (H_(M) =F_(M) /γ).
 4. Amethod of measuring a magnetic field comprising the steps of:(a) placingapparatus having a cavity with a material adapted to produceferromagnetic resonance disposed therein in a magnetic field of acalibrating magnet; (b) transmitting radio frequency energy at apreselected frequency to display video means to generate a referencesignal pulse; (c) coupling radio frequency energy in a predeterminedrange of frequencies including said reference frequency into said cavityto generate first output signals; (d) converting said first outputsignals to video frequencies and transmitting such video signalfrequencies to means for display as video signal pulses; (e) adjustingthe orientation of said material within said cavity until the firstoutput signal pulses are in predetermined alignment with the referencesignal pulse on the video display means, indicating the calibratedorientation of the material in the cavity relative to the magnetic fieldof the calibrating magnet passing through said cavity; (f) substitutinga magnet whose magnetic field is to be measured with the material in acalibrated orientation relative to the magnetic field of thecalibrating-magnet; (g) coupling the radio frequency energy into saidcavity with the material in the calibrated position to generate secondoutput signals for display on said video means; and (h) comparing thesecond video output pulses with the reference signal pulse to provide anindication of the magnetic field properties of the magnet beingmeasured.
 5. A method of measuring magnetic field strength anddistribution comprising the steps of:(a) placing apparatus having aplurality of cavities with material adapted to produce ferromagneticresonance disposed in each cavity in a magnetic field of a calibratingmagnet having known properties (H_(DC)); (b) transmitting radiofrequency energy at a preselected frequency to video display means togenerate a reference signal pulse (f_(c)); (c) coupling modulated andsweeping radio frequency energy in a predetermined range of frequenciesincluding said reference frequency into each of said cavities togenerate first output signals; (d) converting said signals to videofrequencies and transmitting said first video output signals to videodisplay means for display as pulses; (e) adjusting the orientation ofsaid material within said cavities until the first output signal pulsesare in a predetermined array and alignment relative to said referencesignal pulse on the video display means, indicating the calibratedorientation of the material in the cavities relative to the magneticfield of the calibrating magnet passing through each of said cavities,whereby the frequency (f_(c) =γH_(DC)) where γ=the gyromagnetic ratio(2.8 MH_(z) /oersted) and H_(DC) =the applied magnetic field of thecalibrating magnet; (f) substituting a magnet whose magnetic-field(H_(M)) is to be measured with the material in a calibrated orientationfor the calibrating magnet; (g) coupling the radio frequency energy intosaid cavities to generate second output signals, (f_(M)); (h) convertingsaid second output signals to video frequencies and transmitting saidsecond video output signals to video display means for display aspulses; and (i) visually comparing the arrangement of the displayedsecond output signal pulses relative to first output signal pulses toascertain the comparative magnetic field distribution of the magnetbeing measured relative to the field distribution of the calibratingmagnet; and (j) measuring the magnetic field strength by application ofthe equation (f_(M) =γH_(M)) where (f_(M) =the ferromagnetic resonanceof the material in the calibrated orientation in the magnetic-field ofthe magnet being evaluated; γ=gyromagnetic ratio (2.8 MH_(z) /oersted)and H_(M) =applied magnetic field strength of the magnet to be measured.6. The method according to claim 5 wherein said calibrating magnetcomprises an electromagnet having a variable DC voltage source.
 7. Themethod according to claim 5 wherein said material adapted to produceferromagnetic resonance comprises yttrium iron garnet.
 8. The methodaccording to claim 5 wherein said magnet being measured is mountedadjacent to an annular ring of a temperature compensating ferrimagneticmaterial.